Should we be content simply to safeguard as much as we can of the planetary stock of species? Or should we pay equal if not greater attention to safeguarding evolutionary processes at risk (cf. refs. 64-66)? Consider, for instance, biodisparity: to cite Jablonski (49), "If we are concerned with avoiding the loss of particular functional groups, or with maximizing the potential source pool for evolutionary recovery, then biodisparity measures may provide a more appropriate assessment, beyond sheer numbers of taxa, of how priorities should be set."
Following on from these considerations is the question of whether we should seek to maintain the evolutionary status quo by preserving precise phenotypes of particular species, or whether we should prefer to maintain phylogenetic lines that will enable evolutionary adaptations to persist, thereby leading to new species (67, 68). Is it sufficient for us to maintain, for example, just the two elephant species we already have, or should we try to keep open the evolutionary option of further elephant-like species in the distant future?
This is an unusually significant question, with unusually significant implications for conservation strategies. Elephants, along with many other large mammals, are inclined to move around a good deal, a trait that enables them to maintain gene flow across large areas. As a result, their gene pools often tend to be fairly uniform [an elephant in East Africa may not be so different from one 4,000 km away in South Africa (68)]. Regrettably the remaining populations of elephants, substantial and extensive as they are, albeit fragmented and declining fast, are probably already below the minimum numbers to keep open the possibility of speciation (69).
In marked contrast to elephants, with their slow breeding rates, many insect species have immense breeding capacities and rapid turnover rates. These latter attributes offer quick adaptability to environmental shifts, whereupon genetic changes are passed along promptly. These attributes not only leave many insect species well suited to survive the environmental upheavals of human activities, but they offer exceptional scope for speciation in comparatively short order. By contrast, elephants, together with other large-bodied species that reproduce slowly and hence possess restricted capacity for genetic adaptation, will be at an extreme evolutionary disadvantage. Does this factor imply that they should therefore receive all of the greater attention from conservationistsor that, in a triage situation, they should rank lower in our priorities? Although this is a fundamental question, it has hardly been addressed.
An even more important consideration arises concerning those origination centers and radiation lineages that serve as "evolutionary fronts" (67). From the standpoint of future evolution, it is surely more appropriate to safeguard the main potential for diversity generation than to emphasize the primary focus of many current conservation programs, viz. individual taxa and, especially, endemic taxa (70, 71). Much the same applies with respect to those functional groups that increase the potential for evolutionary recovery (49).
All in all, the prospect is that, in the wake of the present biodiversity crisis, we shall find that many evolutionary processes that have persisted throughout the Phanerozoic Eon will be slowed if not depauperized for an extended period. This is not to say, of course, that evolution will come to a halt, or even that speciation will be suspended (except for the large vertebrates). In fact, there may be enough creative disruption in certain environments to foster some extremely rapid microevolutionary changes, attended by (localized?) bursts of speciation. But there will surely be reduced scope for speciation on the scale that has characterized the past many millions of years.
These, then, are some of the issues that we should bear in mind as we begin to impose a fundamental shift on evolution's course. We are "deciding" on evolution's future in virtually a scientific vacuumdeciding all too unwittingly, but effectively and increasingly. Hence the importance of the Colloquium's findings as set out in this special issue of PNAS.
We thank David Jablonski for helpful comments on an early draft of this paper. We also thank the United States National Academy of Sciences and the MacArthur Foundation, Chicago, for funding support. Footnotes
To whom reprint requests should be addressed. E-mail: [email protected]
This introductory paper was presented at the National Academy of Sciences colloquium, "The Future of Evolution," held March 16-20, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA.